As a heat treating method which gives a long life withstanding the rolling fatigue of bearing components, there is a method in which carbonitriding is applied to the surface layer of a bearing component as by adding ammonia gas to an atmosphere RX gas at the time of hardening heating (for example, Japanese Patent Laid-Open Hei 8-4774, and Japanese Patent Laid-Open Hei 11-101247). Using this carbonitriding makes it possible to harden the surface layer and to grow retained austenite in the microstructure, thereby improving the rolling fatigue life.
Carbonitriding requires holding high temperature for a prolonged time for diffusion treatment purposes, resulting in coarsening the structure which, in turn, makes it difficult to improve crack strength, thus leaving room for improvement in this respect. Further, there is also room for improvement in respect of an increase in secular dimensional change rate due to an increase in retained austenite.
On the other hand, securement of a long life withstanding rolling fatigue, improvement in crack strength, and prevention of an increase in secular dimensional change rate can be achieved by alloy design of steel. However, alloy design poses problems, such as an increase in raw material cost.
Future bearing components are required to have characteristics enabling use at high temperatures under higher loading conditions as loads and temperatures in environments for use increase. Therefore, bearing components having a long life in respect of rolling fatigue characteristics and having high degrees of crack strength and dimensional stability will be required.